Cl chapta 7

All variables are of type String, but can be coerced into integers if the example permits it. for instance “12131” looks like a number so it can be used in [ 1212 -eq 414 ] for instance

The shell sees things weirdly

Expansion

echo this is a test will output this is a test as you’d expect. However echo * will NOT do that. Instead it prints every file where you are spaced out

This is because the * gets expanded out into something else:

• When “enter” is pressed, any qualifying character in the command line before the command is run is expanded.
  • echo NEVER saw *, only its expanded result
• Again, but you’ll see even with surrounding words the same problem arises

Wildcards work via pathname expansion

• echo D* would expand D* to every file starting with D separated by spaces
• echo [[:upper:]]* is also a pathname expansion!
• echo /usr/*/share too!

Quirks:

1. echo .* WILL print every file starting with ., however THAT ALSO PRINTS . and .. (the names that refer to the current working directory and parent directory)
2. 
   1. This would be how you list out dotfiles (files that start with .) since usually they are NOT printed / hidden away.

Tilde Expansion

echo ~kido turns into /home/bob

Brace Expansion

echo Front-{A,B,C}-Bank

• Front-A-Back Front-B-Back Front-C-Back is what would be printed
• Also works with ranges: echo {1..6} would print 1 2 3 4 5 6
• And you can nest them: echo {Happy {Camper, Customer}, Red {Rose, Sea}}
  • this would print Happy Camper Happy Customer Red Rose Red Seatodo confirm

acc good stufftodo move outta here

Variables

echo $SUER

• variable no exist = return empty string

Command substitution

echo $(ls)

Quoting

Running echo this is a test, we get this is a test do to word-splitting (the shell removes extra whitespaces from echo’s arguments)

Also running echo You pay $100 would return You Pay 00 since $1 is created like a undefined variable = empty string

Double quotes

• Nullifies the meaning of all but $, \, and `
• I like to think of it as “grouping” a bunch of characters into one concrete “string” This lets us print extra spaces n things.

!! AN IMPORTANT EXAMPLE

echo $(df -h) vs echo "(df -h)

The first one looks like this:

Filesystem Size Used Avail Use% Mounted on tmpfs 1.6G 2.0M 1.6G 1% /run /dev/sda2 94G 19G 71G 21% / tmpfs 7.8G 0 7.8G 0% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock /dev/sda1 975M 6.1M 969M 1% /boot/efi /dev/sdb1 907G 574G 287G 67% /home tmpfs 1.6G 1.8M 1.6G 1% /run/user/1000

and the other looks like this:

Filesystem Size Used Avail Use% Mounted on tmpfs 1.6G 2.0M 1.6G 1% /run /dev/sda2 94G 19G 71G 21% / tmpfs 7.8G 0 7.8G 0% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock /dev/sda1 975M 6.1M 969M 1% /boot/efi /dev/sdb1 907G 574G 287G 67% /home tmpfs 1.6G 1.8M 1.6G 1% /run/user/1000

all organized! Why might that be, though?

Well running just df -h will return a list of things taking space with each line separated by new lines. So $(df -h) returns a bunch of lines separated by delimiters. You remember what doesn’t like delimiters? White-splitting! So before echo is run, the arguments are stripped of their delimiters and printed as a blob.

Using "" will preserve the delimiters, thereby printing out the result nicely!

Single Quotes

Suppresses everything inside of it. String literals if you will

Escaping Characters

echo "I have \$5.00" will NOT treat $ as the operator to fetch variables and will instead treat it as a literal $ Also applicable to !, &, , and other characters that have special meaning to the shell. Also this with \ backslashes

The -e flag for echo

echo -e "It is about time that you...\nfaced the concequences of your actions"

• Allows for control codes

Oh also '\a' does an equivalent thing.